Wide availability, relative inexpense, and functional versatility, have led to the adaptation and use of polyurethane elastomers with specific physical properties in a diverse array of technical applications. One such application is as conductive and semi-conductive members in image forming devices such as electrophotographic printers. As printer art evolves according to the motivation of increased office space efficiency in light of ergonomic considerations, it is desirable to develop more compact image forming devices having a smaller device footprint. As a consequence, there is a need for reduced-geometry elastomeric printer components. However, it has been discovered that necessary size adaptations alter the functioning of the elastomeric components with respect to, for example, required compressive forces, nip widths, triboelectric charging capacity, and rotational torque. Utilization of reduced-geometry components having the same elastomeric properties as conventional larger sized components can result in the occurrence of unacceptable print variations and distortions in the final printed product. Hence, the composition of the elastomers forming the rolls needs to be adapted to confer physical properties which would compensate for the reduced geometry and restore the quality of the final printed product.
In electrophotographic printer applications, the developer roll functions to develop a layer of toner onto a charged photoconducting drum. The toner is metered onto the surface of the developer roll via a doctor blade. The electrical properties of the developer roll assist in the electrostatic transfer of the charged toner from the roller surface to the photoconductor drum. The compositions and properties of conventional developer rolls are disclosed in detail in U.S. Pat. No. 5,707,743 to Janes et al., No. 5,804,114 to Janes et al., No. 5,874,172 to Beach et al., and No. 6,117,557 to Massie II, et al., all of which are incorporated herein by reference. These patents relate to rolls with high surface resistance, processes of manufacturing high surface resistance rolls involving the oxidation of a diene diol such as polybutadiene in the presence of a conductive modifier such as ferric chloride, antioxidant requirements, and the desirability of caprolactone ester polyurethane developer rolls, respectively. These patents also disclose the print performance advantages and electrical properties of two-layered developer rolls and techniques for production of semiconductive rolls in general, and the particular techniques for the production of the resistive surface layer characteristics.
Typically, plasticizers are used to reduce the hardness of polymeric elastomers such as cast polyurethanes. However, plasticizing additives are not chemically bound into the polymer matrix and are therefore free to migrate throughout, including to the surface of a roll formed therefrom. This may cause problems in electrophotographic print applications. At the surface of the developer roll, these migrating additives interact with the toner particles, causing them to coalesce in the nip area of the developer roll and photoconducting drum.
It is known in the polyurethane art that addition of polyols to the polyurethane mixture may lower the hardness of the final polyurethane elastomer. However, these polyols typically result in not only a decrease in hardness, but also in concomitant undesirable changes in other physical properties of the elastomer, such as increased compression set.
Hence, it is desirable to provide soft elastomeric rolls wherein sufficiently low compression set is maintained. It is further desirable that the hardness is decreased via means that do not involve additives with the potential to migrate, and which do not cause unacceptable increases in roll compression set. In addition, it is advantageous to provide a more compliant roll which eliminates the non-uniformities in print performance caused by geometry reduction of the roll and consequential increases in force at the nips between the developer roll and doctor blade and between the developer roll and photoconducting drum.